This invention relates to conducting buffer layers for organic electronic devices and in particular for organic light emitting diodes (OLED)s including flexible, large area OLEDs. The invention also relates to methods for applying the buffer layer.
Organic electronic devices are articles that include layers of organic materials, at least one of which can conduct an electric current. An example of an organic electronic device is an organic light emitting diode (OLED). OLEDs, sometimes referred to as lamps, are desirable for use in electronic media because of their thin profile, low weight, and low driving voltage, i.e., less than about 20 volts. OLEDs have potential use in applications such as backlighting of graphics, pixelated displays, and large emissive graphics.
OLEDs typically consist of an organic light emitter layer and additional organic charge transport layers on both sides of the emitter, all of which are sandwiched between two electrodes: a cathode and an anode. The charge transport layers comprise an electron transporting layer and a hole transporting layer. Charge carriers, i.e., electrons and holes, are injected into the electron and hole transporting layers from the cathode and anode, respectively. Electrons are negatively charged atomic particles and holes are vacant electron energy states that behave as though they are positively charged particles. The charge carriers migrate to the emitter layer, where they combine to emit light.
FIG. 1 illustrates a type of organic light emitting diode. The diode comprises a substrate 12, a first electrode (anode) 14, a hole transporting layer 16, a light emitting layer 18, an electron transporting layer 20, and a second electrode (cathode) 22.
Substrate 12 may be transparent or semi-transparent and may comprise, e.g., glass, or transparent plastics such as polyolefins, polyethersulfones, polycarbonates, polyesters, and polyarylates.
Anode 14 is electrically conductive and may be optically transparent or semi-transparent. Suitable materials for this layer include indium oxide, indium-tin oxide (ITO), zinc oxide, vanadium oxide, zinc-tin oxide, gold, copper, silver, and combinations thereof.
An optional hole injecting layer (not shown) may accept holes from anode layer 14 and transmit them to hole transporting layer 16. Suitable materials for this layer include porphyrinic compounds e.g., copper phthalocyanine (CuPc) and zinc phthalocyanine.
Hole transporting layer 16 facilitates the movement of holes from anode 14 to emitter layer 18. Suitable materials for this layer include, e.g., aromatic tertiary amine materials described in U.S. Pat. Nos. 5,374,489 and 5, 756,224, such as 4,4xe2x80x2,4xe2x80x3-tri(N-phenothiazinyl) triphenylamine (TPTTA), 4,4xe2x80x2,4xe2x80x3-tri(N-phenoxazinyl) triphenylamine (TPOTA), N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-dia (TPD), and polyvinyl carbazole.
Emitter layer 18 comprises an organic material capable of accomodating both holes and electrons. In emitter layer 18, the holes and electrons combine to produce light. Suitable materials for this layer include metal chelate compounds, such as, e.g., tris(8-hydroxyquinolinato) aluminum (AlQ). The emission of light of different colors may be achieved by the use of different emitters and dopants in the emitter layer as described in the art (see C. H. Chen, J. Shi, and C. W. Tang xe2x80x9cRecent Developments in Molecular Organic Electroluminescent Materialsxe2x80x9d, Macromolecular Symposia 1997 125, 1-48).
Electron transporting layer 20 facilitates the movement of electrons from cathode 22 to emitter layer 20. Suitable materials for this layer include, e.g., AlQ, bis( 10-hydroxybenzo(h)quinolinato) beryllium, bis(2-(2-hydroxy-phenyl)-benzolthiazolato) zinc and combinations thereof.
An optional electron injecting layer (not shown) may accept electrons from the cathode 22 and transmit them to the emitter layer 20. Suitable materials for this layer include metal fluorides such as LiF, CsF, as well as SiO2, Al2O3, copper phthalocyanine (CuPc), and alkaline metal compounds comprising at least one of Li, Rb, Cs, Na, and K such as alkaline metal oxides, alkaline metal salts, e.g., Li2O, Cs2O, and LiAlO2.
Cathode 22 provides electrons. It may be transparent. Suitable materials for this layer include, e.g., Mg, Ca, Ag, Al, alloys of Ca and Mg, and ITO.
Polymer OLEDS may be made wherein a single layer of poly(phenylenevinylene) (PPV) or poly(2-methoxy-5-(2xe2x80x2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) functions as layers 16, 18, and 20.
Illustrative examples of known OEL device constructions include molecularly doped polymer devices where charge carrying and/or emitting species are dispersed in a polymer matrix (see J. Kido, xe2x80x9cOrganic Electroluminescent devices Based on Polymeric Materials,xe2x80x9d Trends in Polymer Science, 1994, 2, 350-355), conjugated polymer devices where layers of polymers such as poly(phenylenevinylene) act as the charge carrying and emitting species (see J. J. M. Halls, D. R. Baigent, F. Cacialli, N. C. Greenham, R. H. Friend, S. C. Moratti, and A. B. Holmes, xe2x80x9cLight-emitting and Photoconductive Diodes Fabricated with Conjugated Polymers,xe2x80x9d Thin Solid Films, 1996, 276, 13-20), vapor deposited small molecule heterostructure devices (see U. S. Pat. No. 5,061,569, incorporated by reference, and C. H. Chen, J. Shi, and C. W. Tang, xe2x80x9cRecent Developments in Molecular Organic Electroluminescent Materials,xe2x80x9d Macromolecular Symposia, 1997, 125, 1-48), light emitting electrochemical cells (see Q. Pei, Y.Yang, G. Yu, C. Zang, and A. J. Heeger, xe2x80x9cPolymer Light-Emitting Electrochemical Cells: In Situ Formation of Light-Emitting p-n Junction,xe2x80x9d Journal of the American Chemical Society, 1996, 118, 3922-3929), vertically stacked organic light-emitting diodes capable of emitting light of multiple wavelengths (see U. S. Pat. No. 5,707,745, incorporated by reference, and Z. Shen, P. E. Burrows, V. Bulovic, S. R. Forrest, and M. E. Thompson, xe2x80x9cThree-Color, Tunable, Organic Light-Emitting Devices,xe2x80x9d Science, 1997, 276, 2009-2011).
The present invention relates to a method for adding a buffer layer comprising an intrinsically conducting polymer adjacent to an electrode layer in an organic electronic device such as an OLED to increase performance reliability. For example, the buffer layer may be between the anode layer and hole transporting layer. In particular it can improve the performance of OLEDs that rely upon vapor-coated indium tin oxide as an anode. Adding such a buffer layer to an organic electronic device can reduce or eliminate performance failures such as electrical shorts and non-radiative regions (dark spots).
Unexpectedly, the inventors found they were able to obtain a thin, uniform, smooth polymeric buffer layer by using web coating methods such as microgravure or meniscus coating. Because these coating methods can be used in a continuous process, they enable; adding a beneficial buffer layer to large area OLED substrates and continuous substrate sheets.
For applications requiring large area displays, it becomes more and more difficult to control the number of defects on a substrate or electrode layer. Because the buffer layer of the present invention can minimize the effects that these imperfections have on OLED performance, it enables the production of reliable large area OLEDs, (e g., those having an, area of 250 cm2 or more, or at least one dimension greater than 25 cm).
One aspect of the present invention features an organic electronic device having a buffer layer, comprised of a doped conducting polymer, adjacent to an electrode layer. The conducting polymer may be externally doped or self doped. The electrode may be an anode comprised of indium tin oxide. The organic electronic device may have a flexible substrate, which may be comprised of materials such as, e.g. poly(ethylene terephthalate), polycarbonate, polyolefin, poly(methyl methacrylate), poly(styrene), polyester, polyolefin, polysulfone, fluoropolymer, polyimide, and hybrids, blends, or derivatives of these polymers.
Another aspect of the invention is a method for coating an electrode-coated substrate with a buffer layer by using microgravure or meniscus coating techniques. With such techniques, buffer layers of 500 to 5000 xc3x85 are possible. The smoother the substrate the thinner the buffer layer may be. Preferably, the buffer layers are 500 to 2000 xc3x85 thick.
A further aspect of the present invention provides methods, including a continuous method, of applying the conducting polymer to a large article or sheet of electrode-coated substrate, which may be used in large area organic electronic devices. The method can be used with flexible substrates in a continuous manner, thereby enabling the use of a roll-to-roll manufacturing process. In such a process, the sheet of electrode-coated substrate is continuously dispensed from a roll, passes through the coating area, and wound onto another roll.
As used in this specification:
xe2x80x9cdopantxe2x80x9d means an additive used to modify the conductivity of a polymer; for example, the imine nitrogen of a polyaniline molecule in its base form may be protonated upon exposure of the polyaniline to an acidic solution thereby converting the polyaniline to its conducting form; the acid providing the proton may be referred to as the dopant;
xe2x80x9cexternally dopedxe2x80x9d means a polymer is exposed to an added substance that can change the polymer""s conductivity; for example, an acidic solution can provide a hydrogen ion to dope a polyaniline molecule and can concurrently provide a counterion that is ionically, but not covalently, bonded to the polymer molecule;
xe2x80x9cself dopedxe2x80x9d means that the doping moiety is covalently bonded to the polymer being doped;
xe2x80x9cintrinsically conductingxe2x80x9d means an organic polymer that contains polyconjugated bond systems and that can act as an electrical conductor in the absence of external conductive materials such as metal particles, carbon black, etc.;
xe2x80x9csmall-molecule OLEDxe2x80x9d means a multilayer heterostructure OLED having its nonpolymer layers vapor deposited onto an electrode substrate in a vacuum chamber, wherein xe2x80x9cnon-polymerxe2x80x9d refers to low molecular weight discrete compounds that can be thermally vaporized without causing significant decomposition or other chemical changes;
xe2x80x9cwebxe2x80x9d means a continuous moving support that can carry a substrate past a coater or coating station;
xe2x80x9cweb coating methodxe2x80x9d refers to a coating method suitable for continuously coating a continuous sheet of substrate or a series of discrete substrate articles.
An advantage of at least one embodiment of the present invention is the reduction of defects in an OLED due to a reduction in electrical shorting achieved by coating a buffer layer as described herein on an OLED electrode layer.
Another advantage of at least one embodiment of the present invention is the ability to make a large area buffer-coated anode/substrate using a continuous roll-to-roll process. A roll-to-roll-process for this step can allow for increased production and reduced cost in producing organic electronic devices.